Attention has been recently focused on the ad-hoc networks in which plural wireless terminals communicate each other without relying on particular infrastructure, e.g. base stations, wired networks. In ad-hoc networks, the networks are easily and flexibly formed by using a multi-hop communication through a relay terminal between the terminals that cannot communicate each other directly while using a direct communication between the terminals within the common communication coverage.
As shown in FIG. 7, when a source terminal 21 and a destination terminal 22 initiate a multi-hop communication through the relay terminals 23, and if a system on the shortest path between the source terminal 21 and the destination terminal 22 is communicating (In FIG. 7, a case of communication between the source terminal 31 and the destination terminal 32), it will detour around a communication path. If an ordinary multi-hop communication is performed in this case, hop count is increased and therefore the increment of time to arrive packets and the occupation of channel becomes a problem.
In an ad-hoc network, a distributed ARQ (Automatic Repeat reQuest) method can be employed, and even if signals failed to be received, various kinds of information obtained at the time are stored and a retransmitted signal and the stored signal can be combined (See Japanese patent application laid-open TOKUHYO 2001-518725).
According to a distributed ARQ method where packet transmission from the source terminal through the relay terminals distributed around is repeated until it arrives at the destination terminal, the relay terminals autonomously form a transmission route (that is, without forming a route beforehand) and perform repeating in accordance with packet retransmission by the source terminal. The error is reduced because the relay terminal can receive packets from plural terminals.
In an ordinary system for sending a same packet using a distributed ARQ method, when the signals for the same packet received from two terminals are in opposite phase, the gain of the wireless communication decrease significantly.
As a method for improving the wireless communication characteristics, a method for combining plural signals by applying antenna diversity technique for transmitting packets by plural antennas to distributed terminals in an ad-hoc network has been studied (See Kojima Erina, Fujii Takeo, Kamiya Yukihiro, Suzuki Yasuo “Distributed ARQ using STBC for OFDM ad-hoc networks” Shingaku-gihou, June 2004, RCS2004-77, pp. 7-12). As one of the antenna diversity techniques, STBC (Space Time Block Coding) is known.
In STBC techniques, it is possible to obtain a diversity gain at a receiving side by simultaneously sending differently encoded packets from plural transmitting antennas respectively. It is known to apply a multi-hop communication to Cognitive Radio for effectively using the spatially vacant frequencies. Cognitive Radio technique allows a terminal to autonomously find the vacant frequencies in the frequency band allocated to other systems. e.g. terrestrial broadcasting, and initiate communication.
However even if a multi-hop communication characteristics can be improved by the technique of “Distributed ARQ using STBC for OFDM ad-hoc networks” by Kojima Erina, Fujii Takeo et al., what it detours around is unchanged, the delay and channel occupation problems remain unresolved and there is a limitation to improve the throughput of the entire network. The failure of transmit and receive can be reduced by the technique described in TOKUHYO 2001-518725, however the transmission time cannot be shortened.
A purpose of the present invention is to provide a multi-hop communication method and a multi-hop communication terminal for improving the throughput of the entire network while improving the degradation of the communication characteristics in the circumstance where the packet transmission characteristics degrades because interference exists in the shortest path.
Another purpose of the present invention is to provide a multi-hop communication method, a multi-hop communication terminal and program which can shorten the transmission time while reducing failures of transmit and receive.